1. Field of the Invention
The present invention relates to translucent conductive film-forming coating liquids applied for forming, for example, transparent electrodes of a dispersive type electroluminescent device (hereinafter, sometimes abbreviated as a dispersive type EL device); especially, the translucent conductive film-forming coating liquids having excellent translucency and conductivity, as well as the ability to resist the deterioration of film characteristics such as a resistance value during the EL device production process; translucent conductive film obtained from the translucent conductive film-forming coating liquids; and dispersive type electroluminescent devices which use the translucent conductive film for their transparent electrodes.
2. Description of the Related Art
A dispersive type EL device is a light emitting device driven by alternating voltage, and has so far been used for the back light of liquid crystal displays of a cellular phone and a remote controller, and the like. In recent years, it has been applied to the light-emitting sheet incorporated in the key input part (keypad) of a cellular phone and the like, taking advantage of the thinness (about 0.1 millimeter thick), the flexibility, the in-plane homogeneity of luminance, variety of luminescent colors, low power consumption, and the like.
Generally, the transparent conductive film applied to the transparent electrodes of the dispersive type EL device and the like is formed by an application method using the transparent conductive film-forming coating liquid which contains conductive filler dispersed in a solvent containing a binder.
As the conductive fillers used for such transparent conductive film-forming coating liquids, there are, conventionally, oxide type fillers, such as an indium-tin oxide (hereinafter also called “ITO”) and an antimony-tin oxide (hereinafter also called “ATO”). and ITO is especially used widely, since its resistance value is lower than ATO and the like.
By the way, the smaller the content of the conductive filler used in the above-mentioned transparent conductive film-forming coating liquid, the more preferable. This is because the optical absorption of the conductive oxide, the conductive filler, is by far larger than that of the binder consisting of transparent resin, one of the coating components.
Accordingly, the visible light transmittance of the film will be increased by reducing the amount of the conductive oxide filler relative to the binder to be as small as possible, within the range that a conductive film with a low resistance value can be obtained.
Due to these reasons, the film with a lower resistance value is advantageously obtained by addition of smaller amount of acicular or scaly conductive filler than spherical or granular conductive filler.
As a method for obtaining the above scaly oxide powder, for example, according to the description of Patent document 1, there is a procedure that, after freezing a colloidal dispersion liquid of an inorganic oxide fine particle, a hydrated inorganic oxide fine particle or the like, and allowing inorganic oxide fine particles or hydrated oxide fine particles to deposit in the gaps between the crystal faces of the solvent of the colloidal dispersion liquid, the fine particles are dried to remove the solvent, and further calcinated in the case of the hydrated oxide fine particles.
As methods for obtaining the above acicular oxide powder, for example, a procedure according to the indication of Patent document 2 of thermally decomposing acicular tin oxalate to obtain acicular tin oxide, or a procedure according to the indication of Patent document 3 of thermally decomposing white acicular indium compound powder recovered from the concentrated slurry of indium nitrate solution heated at high temperature, to obtain acicular indium-tin oxide, have been known.
As a conductive film-forming coating liquid that uses the above-mentioned conductive filler, a paste using acicular ITO powder has been known, as indicated such as in Patent documents 4 to 6. Since a conductive film-formed by using the conductive film-forming coating liquid containing such conductive oxide acicular powder has a higher haze value (large scattering rate) than the so-called transparent conductive film, it is called a translucent conductive film.
The above-mentioned translucent conductive film is not transparent but has excellent translucency with small light-absorption, and thus is applicable enough as transparent electrodes of the dispersive type EL device and the like.
However, when the (translucent) conductive film-forming coating liquid (paste) adopting the conventional binder resin according to the descriptions of the above Patent documents 4 to 6, was going to be used as the transparent electrode of the dispersive type EL device, the following problem turned out to occur: if a phosphor layer, a dielectric layer, or the like is lamination-printed on the above-mentioned translucent conductive film, during the production process of the EL device the film characteristics (the resistance value, and the like) of the translucent conductive film deteriorates.
Patent document 1: Laid-open Japanese patent publication No. S62-3003,
Patent document 2: Laid-open Japanese patent publication No. S56-120519,
Patent document 3: Laid-open Japanese patent publication No. H6-293515,
Patent document 4: Laid-open Japanese patent publication No. H6-309922,
Patent document 5: Laid-open Japanese patent publication No. H9-35873,
Patent document 6: Laid-open Japanese patent publication No. H11-273874.